Carousel: allow to filter EXIF tags that will be displayed in lightbox

[eliorivero]

Fixes #861

Changes proposed in this Pull Request:

• display "Copyright" tag
• allow to filter the EXIF tags that will be included.
  They can be added now using something like this:

  function jp_custom_exif_info( $js ) {
  	// Add tag to list
  	$js['meta_data'][] = 'credit';
  
  	// Add translation for tag
  	$js['credit'] = __( 'Credit', 'jetpack' );
  
  	return $js;
  } 
  add_filter( 'jp_carousel_localize_strings', 'jp_custom_exif_info');

  The filter also allows to redefine the tags that will be displayed by overwriting the list with a full array instead of appending an item like in the previous example:

  function jp_custom_exif_info( $js ) {
  	// Overwrite with a new list
  	$js['meta_data'] = array( 'copyright', 'credit' );
  
  	// Add translation for tag
  	$js['credit'] = __( 'Credit', 'jetpack' );
  
  	return $js;
  } 
  add_filter( 'jp_carousel_localize_strings', 'jp_custom_exif_info');

• the styles will prevent the exif data blocks from being incorrectly stacked
  With previous styles, the last block is incorrectly floated towards right
  
  With the updated styles, the last block is correctly placed towards left
  

Testing instructions:

• use an image with copyright field in the EXIF metadata

Proposed changelog entry for your changes:

Carousel: now displays copyright EXIF information if it's available and allows to define custom EXIF information to display.

[thingalon]

LGTM, and looks like it works: